1. Field of the Invention
The present invention relates to a wireless communications system, and more specifically to an adaptive antenna control technology in a wireless communications system for use in an environment in which communications by other systems are also established, and a mobile communications system in which high-speed and high-quality data communications are required.
2. Description of the Related Art
In a wireless communications system such as a wireless LAN system, etc., a communications system such as Bluetooth, etc. is commonly available by a number of users. Such an efficient communications system can be a multiple access system. This multiple access system is widely used as a mobile communications system.
In a wireless communications system in which a transmission data sequence is converted in parallel into a plurality of data sequences, and a plurality of parallel data sequences are transmitted in parallel by wireless using a plurality of carriers having different frequencies, for example, an OFDM (orthogonal frequency division-multiplexing) system, different pieces of information can be transmitted using a plurality of carriers. Additionally, since each subcarrier is low in transmission speed, it is not subject to frequency selective fading, and is resistant to a delay by inserting a guard interval as the same data as those coming later in time. Therefore, it is suitable for a wireless communications system in the future. However, when it receives a wave exceeding a guard interval, an interference wave from another system, and an interference wave with a Doppler shift, the properties of the system can be exceedingly degraded.
There are the following conventional technologies to prevent the degradation of properties due to the above-mentioned interference waves
Patent Literature 1
Japanese Patent Application Laid-open No. 8-331025 “Adaptive Interference Cancellation Receiver”
Literature 1 of Unpatented Case
J. CHENG, et al. “Adaptive Beamforming of ESPAR Antenna Based on Steepest Gradient Algorithm IEICE Trans. COMMUN., E84-B, 7, 2001.”
The Patent Literature 1 discloses an adaptive interference cancellation communication apparatus including a unit, provided for each of a plurality of reception antenna, for weighting a received signal, and a unit for composing a weighted signal.
The Literature 1 of the Unpatented Case discloses an adaptive beam forming technology using the steepest gradient algorithm in the adaptive antenna in which each antenna element has a load reactance.
The conventional technology of the interference wave suppression system for suppressing the reception of the above-mentioned interference waves is described below in more detail by referring to FIGS. 1 through 4. Normally, it is effective to adjust the length of an antenna element so as to change the directivity of an antenna. A desired wave to be received can be received by adjusting the length of an antenna element, and the directivity of an antenna can be controlled not to receive an interference wave.
However, it is actually difficult to mechanically change the length of an antenna element. In the adaptive antenna system, the directivity is changed using a method different from the above-mentioned method so that the null of the directivity of an antenna can be controlled to face an interference wave.
FIG. 1 shows an adaptive control system which is an adaptive antenna system for changing the directivity by changing a weight value by providing a weight unit which weights signals from a plurality of antenna elements and a composing circuit which composes a weighted signal, and in which the weighting process is referred to as digital beam forming performed in a processing area of a digital signal.
A signal from each antenna element 51 is converted into a digital signal by an A/D converter 53 through a high frequency front end (RF F/E) 52, for example, a mixer and a band pass filter, weighted by a weight unit 54, and then composed by a composing circuit 55. The weight value by each weight unit 54 is controlled by a weight control circuit 56. The weight control circuit 56 controls a weight value such that the interference-to-noise ratio (INR) in the output signal of the composing circuit 55 can be decreased to face the null of the directivity of an antenna to an interference wave.
In the system shown in FIG. 1, the high frequency front end 52, the A/D converter 53, etc. are required for each antenna element, thereby upsizing the entire circuit, and increasing the power consumption. To solve this problem, an adaptive control system shown in FIG. 2 can be useful in downsizing the circuit and reducing the power consumption. In FIG. 2, a weight unit 62 for weighting a signal from an antenna element 61 is provided in a high frequency area, and after a weighted signal is added by an addition circuit 63, it is supplied to a high frequency front end 65. The output of the high frequency front end 65 is converted into a digital signal by an A/D converter 66. Using the signal, a weight control unit 64 controls a weight value of each weight unit 62, thereby performing adaptive control on an antenna.
FIG. 3 shows the system of performing adaptive control by changing a variable reactance value as a load of a no-feed antenna element in the antenna elements instead of changing the weight value for a signal from an antenna element. In FIG. 3, a signal from a feed antenna element 71 is converted into a digital signal by an A/D converter 76 through a high frequency front end 75.
A variable reactance circuit 73 is connected as a load to each no-feed element 72. Using the output of the A/D converter 76, the directivity of the entire antenna can be changed by a variable reactance control unit 74 changing the variable reactance value of each variable reactance circuit 73.
The output signal of the feed antenna element 71 receives the influence of each other combined electromagnetic fields of the surrounding no feed elements 72, and the null of the directivity of an antenna can face an interference wave by the adaptive control of the variable reactance value.
For example, when an adaptive antenna is controlled in the OFDM wireless communications system using the system shown in FIG. 3, a carrier not practically used in the actual communications in a number of carriers, that is, a virtual subcarrier element, is observed, the reactance values of all no-feed elements are perturbed, and the operation of updating a variable reactance value is repeated such that the interference-to-noise ratio can be reduced, thereby performing the adaptive control on an antenna.
FIG. 4 is an explanatory view of the conventional technology of the perturbation system in the above-mentioned adaptive control. In FIG. 4, continued is the operation in which the variable reactance value for the first elements 1 in the variable reactance values corresponding to a plurality of no-feed elements is perturbed in the first symbol, the variable reactance value for the element 2 is perturbed in the next symbol, and the variable reactance value for the element 3 is perturbed in the third symbol.
That is, in the system shown in FIG. 4, when there are M no-feed antenna elements, an M-symbol time is required to perturb them, and also 1-symbol unperturbed data is required for evaluation of perturbation. Therefore, a total of M+1 symbol time is required to update the variable reactance value of the antenna.